Foams and other highly porous materials with a cellular structure are known to have many interesting combinations of physical and mechanical properties, such as high stiffness in conjunction with very low specific weight or high gas permeability combined with high thermal conductivity. Among man-made cellular materials, polymeric foams are currently the most important ones with widespread applications in nearly every sector of technology. Metals and alloys may also be produced as cellular materials or foams.
There are many ways to manufacture cellular metallic materials. Some methods are similar to techniques used for foaming aqueous or polymer liquids, whereas others are specially designed by taking advantage of characteristic properties of metals such as their sintering activity or the fact that they can be electrically deposited. The various methods can be classified according to the state in which the metal is processed. The four “families” of processes are as follows: (i) from liquid metal, (ii) from solid metal in powdered form, (iii) from metal vapor or gaseous metallic compounds, and (iv) from a metal ion solution.
Power metallurgy is a method of forming conventional closed cell foams where the starting materials are metal powders and where the actual foaming takes place in the liquid state. The production process begins with the mixing of metal powders, which can be made up of elementary metal powders, alloy powders or metal powder blends in the presence of a blowing or foaming agent. Afterwards the mix is compacted to yield a dense, semi-finished product. In principle, the compaction can be done by any technique that ensures that the blowing agent is embedded into the metal matrix without any notable residual open porosity. Examples of such compaction methods are hot uniaxial or isostatic compression, rod extrusion or powder rolling. Which compaction method is chosen depends on the required shape of the precursor material. Rectangular profiles with various cross-sections may be made, from which thin sheets can then be formed by rolling. The manufacture of the precursor has to be carried out carefully because any residual porosity or other defects may lead to poor results in further processing.
Heat treatment at temperatures near the melting point of the matrix material is the next step in the powder metallurgy process. The blowing or foaming agent, which is homogeneously distributed within the dense metallic matrix, decomposes at these temperatures. The released gas forces the compacted precursor material to expand, thus forming its highly porous structure.
FIG. 1 shows expansion curves of an aluminum/TiH2 powder compact when foamed at 750° C. The volume of the expanding foam is shown as a function of time together along with some morphologies in various stages of expansion (including the unexpanded precursor which is 9 mm in height and 32 mm in diameter). An expansion maximum is shown which corresponds to a fairly uniform foam morphology, after which the foam collapses. The degree of maximum expansion, and therefore, the density of the solid metal foam, can be controlled by adjusting the content of the blowing agent and several other foaming parameters, such as temperature and heating rates.
The method is not restricted to aluminum and its alloys. Tin, zinc, brass, lead, gold and some other metals and alloys can also be foamed by choosing appropriate blowing agents and process parameters. The most common alloys for foaming are pure aluminum or wrought alloys. Casting alloys such as AlSi7Mg (A356) and AlSi12 are also frequently used because of their low melting point and good foaming properties.
U.S. Pat. No. 5,302,414 to Alkhimov et al., herein incorporated by reference in its entirety, discloses a cold gas-dynamic spraying method for applying a coating to an article by introducing into a gas, particles of a powder of a metal, alloy, polymer or mechanical mixture of a metal and an alloy. The gas and particles are formed into a supersonic jet having a temperature considerably below a fusing temperature of the powder material and a velocity of from about 300 to about 1,200 m/sec. The jet is then directed against an article of a metal, alloy or dielectric, thereby coating the article with the particles.
U.S. Pat. No. 6,408,928 to Heinrich et al., herein incorporated by reference in its entirety, discloses an apparatus for producing expandable metal, comprising (1) means for feeding a powder mixture containing at least one metal powder and at least one blowing agent in powder form; (2) means for producing a compact body from the powder mixture; and (3) means for heating the compact body to a temperature equal to or above the breakdown temperature of the blowing agent. The cold-gas spray apparatus can be used to form metal foams obtained from the foamable metal bodies.
U.S. Pat. No. 6,464,933 to Popoola et al., herein incorporated by reference in its entirety, discloses a method of fabricating a foamed metal structure using a supply of metal particles. The method comprises the steps of (a) introducing a supply of powder metal particles and foaming agent particles into a propellant gas to form a gas/particle mixture; (b) projecting the mixture at or above a critical velocity of at least sonic velocity onto a metallic substrate to create a deposit of pressure-compacted metal particles containing the admixed foaming agent; and (c) subjecting at least the coating on said substrate to a thermal excursion effective to activate expansion of the foaming agent while softening the metal particles for plastic deformation under the influence of the expanding gases.
A need exists for an improved method to form a metallic foam where the powder metallurgy steps of mixing and compaction are eliminated. A need also exists for a method to form a surface layer of a metallic foam structure on preexisting surfaces.